Preheat your oven to 400°F (200°C) and line a 12-cup muffin tin with paper liners.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, baking powder, ground cinnamon, baking soda, and salt.

In another bowl, blend together sour cream, melted unsalted butter, eggs, and vanilla extract until smooth.

Gently fold the wet ingredients into the dry mixture until just combined.

Carefully fold in the diced rhubarb, ensuring even distribution.

Spoon the batter into the lined muffin cups, filling each about three-quarters full.

In a small bowl, mix together vanilla sugar and cinnamon, then sprinkle over the muffin tops.

Bake for 18-22 minutes, or until golden brown and a toothpick inserted comes out clean.

Let muffins cool for 5-10 minutes in the tin, then transfer to a wire rack to cool completely.